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Understanding Crystallography and Minerals in Basic Geology

Explore the world of crystals and minerals in geology. Learn about crystal structures, symmetry elements, transparency, and properties of minerals. Discover the fascinating world of crystallography.

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Understanding Crystallography and Minerals in Basic Geology

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  1. Basic Geology 1. Crystallography

  2. Crystal is a convexpolyhedron. Crystal is a solidmaterialinwhichatoms and/orionsarearrangedandchemicallyboundedin a regular and repeatinglong-rangepattern.

  3. MINERAL is a naturallyoccuring crystallinesolidwith a definitechemicalcomposition.

  4. Naturallyoccuring, i.e. it is formedwithout human actionorintervention, butitmay be biotic(biomineralization).

  5. Crystallinesolids, butsomemineralsareamorphous (mineraloids).

  6. - Definitechemicalcomposition, butit is notnecessarily fixed. Olivine – (Fe,Mg)2SiO4

  7. Spot repeatedbytranslating parallel toavector row of dots Repeatedtranslations parallel toa and b planelattice Repeatedplanelatticesoneabove the other spacelattice Unit cell: thesmallest group of particles in the material that constitutes the repeating pattern. The unit cell completely defines the symmetry and structure of the entire crystal lattice, which is built up by repetitivetranslationof the unit cell along its principal axes.

  8. Triclinic Tetragonal Hexagonal Monoclinic Trigonal Isometric Orthorhombic

  9. SYMMETRY ELEMENTS Rotational symmetryinvolvesrepeating a motifbyaset of uniform rotationabout an axispassingthrough the crystal. The entirecrystalcanbe produced 1-, 2-, 3-, 4- or 6-fold rotations. If a crystal has inversionsymmetry, any line drawnthrough the originwillfindidenticalfeaturesequidistantfromtheorigin on oppositeside of the crystal. Inversionsmove faces to a reversal of position; the old and new positions lie upon a line, at the middle of which is the centre of inversion. Improperrotations are rotations followed by inversions (called rotoinversions). Reflections exchange the parts of the crystal on the two sides of a plane of symmetry (mirror plane)so the pattern on oneside is a mirror image of the pattern on the other.

  10. A – rotation (2, 3, 4, 6) B – rotoinversionorimproperrotation(3,4) C – inversion D – reflection

  11. The amount of light able to be passed through a mineral determines itstransparency. • Opaqueminerals do not let any light through. • Translucent minerals partially let light pass through. • Light is able to pass through transparentminerals.

  12. Index of refraction (n) n=Vv/Vm Vvvelocityinvacuum Vmvelocityin the material Doublerefractionorbirefrigence

  13. ISOTROPIC ANISOTROPIC Propertiesaredirectionindependent. Propertiesaredirectiondependent. A singlerefractive index. More thanonerefractiveindices. No birefringence. Birefringence. Cubiccrystals and amorphousmaterials. Allcrystalsexceptcubicones.

  14. POLARIZING MICROSCOPE

  15. Unpolarizedlightvibratesinalldirections. b) Polarizedlightvibratesin a singleplane

  16. Planelightview Crossedpolarview

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